As mobile Internet services increase rapidly, in several foreseeable years in the future, existing mobile communication spectrum resources will not be able to meet a requirement of a rapid increase of mobile data. At present, most low-band spectrum resources (for example, frequency bands below 3 GHz) that are suitable for mobile communication have been allocated. However, in frequency bands between 3 GHz and 300 GHz, a large quantity of spectrum resources have not been allocated for use. According to a definition by the International Telecommunication Union (International Telecommunication Union, ITU), the 3-30 GHz band is called a super high frequency (Super High Frequency, SHF) band, and the 30-300 GHz band is called an extremely high frequency (Extremely High Frequency, EHF) band. Because the SHF band and the EHF band have a similar propagation characteristic (a relatively high propagation loss) with a wavelength ranging from 1 millimeter to 100 millimeters, the bands between 3 GHz and 300 GHz are collectively referred to as a millimeter-wave frequency band. Extending to the millimeter-wave frequency band to explore usable frequency resources has become a common thinking of the industry.
By making use of characteristics of the millimeter-wave frequency band, in a millimeter-wave communications system, a millimeter-wave macro base station transmits control plane information and user plane information by using a millimeter-wave frequency band with a relatively low frequency (which is referred to as a low frequency band below and is generally a frequency band below 6 GHz, such as 3.5 GHz, or 5 GHz), and may cover a relatively large area. Both a small cell and a macro base station can transmit user plane information by using a relatively high millimeter-wave frequency band (which is referred to as a high frequency band below and is generally a frequency band above 6 GHz, such as 28 GHz, 38 GHz, or E-Band). In coverage of the macro base station, a millimeter-wave small cell is used for relay transmission, and multiple millimeter-wave small cells are deployed for hotspot coverage, where the millimeter-wave small cell covers a relatively small area by using a millimeter-wave frequency band with a relatively high frequency. A terminal in coverage of the millimeter-wave macro base station and in coverage of the millimeter-wave small cell may communicate with both the millimeter-wave small cell and the millimeter-wave macro base station by using one or more millimeter-wave frequency bands. In this way, a system capacity can be effectively improved, and real-time transmission of control signaling can be effectively guaranteed.
That a relay technology is used as a key technology component is proposed in both Long Term Evolution-Advanced (Long Term Evolution-Advanced, LTE-A) and Wireless Metropolitan Area Network-Advanced (Wireless MAN-Advanced). LTE-A is used as an example. A relay node (Relay Node, RN) implements all functions of a base station, and in a process of communicating with a donor evolved NodeB (Donor eNB, DeNB), needs to complete air interface information processing work similar to that of a terminal UE.
However, in an existing relay transmission solution, processing by a relay node is relatively complex. Air interface information processing is used as an example. Processing at various layers including Packet Data Convergence Protocol (Packet Data Convergence Protocol, PDCP), Radio Link Control (Radio Link Control, RLC), Media Access Control (Media Access Control, MAC), and the Physical Layer (Physical Layer, PHY) protocol needs to be performed. According to 3GPP evaluation, a required processing time is about 1.5 ms, and therefore, a time that needs to be consumed for one time of forwarding is longer than 3 ms. In requirements newly discussed by the ITU for a next-generation mobile communication system, it is proposed that a user plane processing delay that does not exceed 1 ms needs to be supported. Therefore, the existing solution cannot meet the delay requirement.